The present invention relates to fiber optic sensors. Specifically, the invention relates to fiber optic current sensors and their signal processing electronics.
The interferometric Fiber Optic Sensor (FOS) is an established technology for accurately measuring angular rotation (interferometric Fiber Optic Gyroscope, FOG) and magnetic fields (interferometric or polarimetric fiber optic current sensor, FOCS). It will be understood that the FOCS does not directly sense an electric current, but rather the effect of a magnetic field produced by that current. Because the FOS is an optical, solid state design with no moving parts, it can be used for long life, high reliability applications such as vehicle navigation and remote sensing of electric currents.
The fundamental working principle behind the FOS is the Sagnac effect for rotation sensors (FOG) and the Faraday effect for current sensors (FOCS). In the FOG measuring rotation, two counter-propagating light waves traversing a loop interferometer acquire a phase difference when the loop is rotated about its axis. In a FOCS measuring an electric current flowing in a wire that passes through the coil plane, the phase difference is produced by the magnetic field associated with the current. Depending on the configuration of the optical path of the light waves, the FOS may incorporate interferometric or polarimetric phase modulation. Accurate measurement of the phase difference induced by the rotation or current requires the parasitic phase differences, which can vary with the environment, be suppressed. For this reason, the principle of optical reciprocity is used to select portions of the counter-propagating waves which pass through the interferometer or polarimeter along a common path. In the following, xe2x80x9cinterferometerxe2x80x9d is meant to refer to both interferometric or polarimetric devices. Variations induced in the system by the environment change the phase of both waves equally and no difference in phase delay results; the sensor is environmentally stable.
In a conventional interferometric fiber optic gyroscope (FOG), the light emitted from a suitable light source passes through a first 3 dB coupler where half of the light is dissipated, and half is sent into the interferometer through the polarizer. A second 3 dB coupler splits the light into two approximately equal intensity, counter-propagating beams which traverse the coil. The two light beams then recombine at the second coupler where they interfere. This combined light beam then passes through the polarizer a second time in the opposite direction, and half of the light is directed to the detector by the first coupler. The first coupler is not part of the optically reciprocal Sagnac interferometer. Its sole purpose is to direct some of the returning light into a photodetector and to minimize direct coupling of light energy from the source into the detector. An optical splitting ratio of 3 dB is selected for the couplers to maximize the optical power incident on the detector. This leads to an inherent 6 dB of system loss since this coupler is passed twice.
In a conventional interferometric fiber optic current sensor (FOCS), the light beam also passes through a first directional coupler that isolates the optical detector, and then through a polarizer which produces linearly polarized light. The linearly polarized light beam is then split in two by the second directional coupler, with one beam being directed into one end of a sensing coil comprising loops of non-birefringent fiber. The other of the two light beams is directed through a phase modulator into the other end of the sensing coil. Before entering the sensing coil, each of the linearly polarized light beams passes through a respective quarter wave plate and emerges therefrom as circularly polarized light, which is the light entering the sensing coil. Light emerging from the two fiber ends of the sensing coil passes again through the quarter wave plate, producing linearly polarized return light. The return light is recombined by the second directional coupler and the intensity of the recombined light is detected by an optical detector.
Another type of conventional FOCS is based on a reflective current sensing coil which simplifies the design by eliminating the second directional coupler. The orthogonally linearly polarized light beams pass through a quarter wave plate and emerges therefrom as counter-rotating circularly polarized light prior to entering the sensing coil. Upon reflection at the end of the fiber, the sense of rotation of the two light waves are reversed and the light travels back through the sensing region, whereafter the light is converted back to linearly polarized light. The intensity of the recombined light is detected by an optical detector. If a birefringence modulator is used instead of a phase modulator, a 45xc2x0 splice has to be inserted before the light enters the modulator.
The phase modulator can be, for example, a piezo-electric transducer (PZT). Other methods of modulating the phase difference, for example, electro-optic material such as lithium niobate can be used. If an integrated optics assembly (IOC) is used for modulation, then a Y-junction power splitter may be included instead of the directional coupler connected to the sensing coil. This phase modulation serves two purposes. One is to dynamically bias the interferometer to a more sensitive operating point and also allow the determination of rotation sense. The other is to move the detected signal from direct current (DC) to alternating current (AC) in order to improve the accuracy of the electrical signal processing. With sinusoidal phase modulation in an open-loop signal processing configuration, the interferometer output signal is an infinite series of sine and cosine waveforms whose maximum amplitudes are Bessel functions related to the phase modulation amplitude. The maximum amplitudes of the Bessel functions are proportional to the sine (odd harmonics) and cosine (even harmonics) of the measured quantity. The fundamental signal is located at the applied modulation frequency with subsequent even and odd harmonic signals. Many signal processing approaches have been proposed which use ratios of the fundamental and the three lowest order harmonic signals amplitudes to detect rotation rate and/or magnetic field, while at the same time maintaining a stable, linear output scale factor. However implementation of these approaches in analog and/or digital electronic hardware is complex and expensive. A much simpler signal processing design, which is not affected by an error in the relative amplitude of the sensor harmonic signals is therefore desired.
Scale factor linearity (i.e. measured current or magnetic field versus the applied current or magnetic field) is maintained due to the intrinsic linearity of the Sagnac and Faraday effect for small measured current or magnetic field values. At higher rate or currents, during environment changes (i.e. temperature, vibration, etc.) and over the life time of the sensor, the linearity can be maintained using conventional signal processing techniques.
It would therefore be desirable to provide a fiber optic current sensor with a smaller number of optical components which can be produced more easily and less expensively. It would also be desirable to provide an electronic system for processing the current sensor output signal in order to maintain a constant scale factor during environment changes.
The present invention is directed to a reduced minimum configuration (RMC) fiberoptic current sensor (FOCS) system for measuring a magnetic field, in particular a magnetic field induced by an electric current. According to one aspect of the invention, the RMC FOCS system includes a fiber sensing coil; a light source having a front output and a back output and emitting light with an associated light source intensity; an optical coupler, which may also include a polarizer, disposed between the front output and the coil and receiving the light from the light source, the coupler creating two linearly polarized light beams of substantially equal intensity; a first quarter wave plate disposed proximate to a first end of the fiber sensing coil and receiving a first of the two linearly polarized light beams and converting the first linearly polarized light beam into first circularly polarized light propagate through the sensing coil in a first direction; a second quarter wave plate disposed proximate to a second end of the fiber sensing coil and receiving a second of the two linearly polarized light beams and converting the second linearly polarized light beam into second circularly polarized light propagate through the sensing coil in a second direction opposite the first direction, the first and second circularly polarized light passing though the sensing coil experiencing a differential phase shift caused by a magnetic field or current flowing in a conductor proximate to the sensing coil; the fiber sensing coil supplying phase-shifted circularly polarized return light to the first and second quarter wave plates, the first and second quarter wave plates converting the phase-shifted circularly polarized return light back to linearly polarized return light, the coupler combining and interfering the linearly polarized return light into a combined light beam; and a light detector operatively coupled to the back output of the light source, the light detector detecting and providing an output signal in response to the combined light beam transmitted through the light source.
According to another aspect of the invention, the reduced minimum configuration (RMC) fiber optic current sensor (FOCS) system includes a fiber optic sensing region with an optical fiber; a light source having a front output and a back output and emitting light with an associated light source intensity; and an optical path operatively connecting the front output of the light source with the fiber optic sensing region for transmitting two linearly polarized light beams from the light source along the optical path. At least one quarter wave plate is disposed between the optical path and the sensing region for converting the two linearly polarized light beams into two opposing circularly polarized light beams propagating through the sensing region, with the two opposing circularly polarized light beams which propagate though the sensing region experiencing a differential phase shift caused by a magnetic field or current flowing in a conductor proximate to the sensing coil. The fiber sensing region may include a reflector located at an end portion of the fiber sensing region that reflects the circularly polarized light beams, supplying phase-shifted circularly polarized return light to the at least one quarter wave plate which converts the phase-shifted circularly polarized return light back to interfering linearly polarized return light beams. A light detector which is operatively coupled to the back output of the light source detects the interfered return light beams transmitted through the light source and provides an output signal in response to the interfered return light beams transmitted through the light source.
Embodiments of the RMC FOCS system may include one or more of the following features. The RMC FOCS may include a polarizer disposed between the front output and the sensor coil, with the polarizer polarizing the light emitted from the light source and the return light beam. In a reflective RMC FOCS, a 45xc2x0 twist may be inserted between the polarizer and the sensor coil. An optical phase or birefringence modulator may be coupled to the sensor coil; an oscillator, which controls the modulation amplitude, may be coupled to the modulator. An electrical amplifier which receives the output signal may be coupled to the detector, with a direct current block, a rectifier, an integrating comparator, and light source drive means coupled to the amplifier, for controlling the associated light source intensity. Electrical signal processing means may also be coupled to the amplifier for processing the output signal and providing an output value correlated with the magnetic field or current.